Tarnish resistant silver based dental casting alloy having superior improved ductility and work hardening characteristics

ABSTRACT

An alloy comprising 10-60% Pd, 0.2-10% of at least one member of the group consisting of Cr, Fe, In, Sn and Zn, the balance silver having improved tarnish resistance meeting the Vickers hardness requirements of the American Dental Association Specification 5 for dental castings, which has superior ductility and work hardening characteristics.

United States Patent 11 1 Ingersoll Dec. 30, 1975 [54] TARNISH RESISTANT SILVER BASED 2,070,271 2/1937 DENTAL CASTING ALLoY HAVING SUPERIOR IMPROVED DUCTILITY AND 3,155,467 11/1964 Yamamoto et al. 75/173 R X WORK HARDENING CHARACTERISTICS 3,411,900 11/1968 Roeder et a1. 75 173 c [75] Inventor: glygle E. Ingersoll, Tonawanda, FOREIGN PATENTS QR APPLICATIONS 628,572 4/1936 Germany 75/173 R [73] Assignee: Williams Gold Refining Company Incorporated, Buffalo, NY. Primary ExaminerL. Dewayne Rutledge Assistant Examiner-E. L. Weise [22] Filed 1974 Attorney, Agent, or Firm-Christel & Bean [21] Appl. No.: 494,565

[57] ABSTRACT 52 U.S. Cl 75 173 R; 75/134 N; 75/172 G An alloy comprising 10-60% 2-1 of at least [51] Int. Cl. C22C 5/06 one member of the g p consisting of CT, [58] Field of S r h 75/173 R, 172 G 134 N and Zn, the balance silver having improved tarnish resistance meeting the Vickers hardness requirements of [56] Ref r n Cit d the American Dental Association Specification 5 for UNITED STATES PATENTS dental castings, which has superior ductility and work 1,229,037 6/1917 Cooper 75 173 R hardemng charactensncs' 1,999,866 4/1935 Capillon et a1. 75/172 G 6 Claims, No Drawings US. Pat. application Ser. No. 494,566, filed of even date by the same inventor, is directed to a related silver based alloy useful in the dental and jewelry arts.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to silver palladium alloys employed in the dental and jewelry arts which are resistant to tarnishing and which have superior ductility and work hardening characteristics and which meet the requirements of the American Dental Association (ADA) Specification 5 for dental castings.

2. Description of the Prior Art Gold alloys have long been used in the dental arts for restorative appliances. Soft alloys of gold with small amounts of silver and/or copper were used in earlier times .for simple one-surface inlays, where the restoration was surrounded by tooth structure. However, for two or three surface inlays or full crowns, soft alloys of 20K or 22K were not strong enough. More complicated alloys were used to meet the strength requirements. Copper was employed as a strengthening ingredient, silver was used to dilute the gold and produce a cheaper alloy, zinc and/or indium was used as an oxygen scavenger. Palladium and platinum were also used for strengthening, color control and corrosion resistance, the palladium also serving to lower the cost.

As the price of gold increased, cobalt and nickel base alloys were introduced for partial denture use. Silver alloys of platinum or palladium were tried but did not prove successful. However, it was difficult to make precision casts for crowns and inlays from cobalt and nickel base alloys and gold alloys continued to be used for these purposes.

Efforts to substitute silver base alloys for gold base alloys continued. US. Pat. No. 2,198,400 shows an alloy containing silver, palladium and hardened with copper and including a small percentage of gold, with zinc as a scavenger. However, as US Pat. No. 2,259,668 points out, copper present in large quantities causes brittleness. Since dental alloys are furnished to the dentist or dental laboratory in the form of wrought pieces, they should be workable by rolling. The prior art alloys, which contain copper as a strengthener, work harden very rapidly. Thus, only a small amount of reduction by rolling can be achieved after each strain relief treatment. In addition, brittle alloys are often undesirable from the point of view of dental technique. In many instances, the dentist must burnish crown margins to give better adptation after cementation. Brittle margins are likely to break off during the burnishing operation.

A further problem with silver dental alloys is the ease with which they tarnish. US Pat. No. 2,196,303; 2,196,304 and 2,259,668 state that zinc and cadmium improve the tarnish resistance of the silver alloys shown, all of which contain large percentages of copper.

SUMMARY OF THE INVENTION The present invention comprises alloys of silver and palladium containing at least one member of the group of Cr, Fe, In, Sn and Zn having good ductility and work hardening characteristics and improved tarnish resistance.

OBJECTS OF THE INVENTION An object of the invention is to provide a silver based alloy having an improved tarnish resistance.

Another object of the invention is to provide a silver based alloy having sufficient strength to be employed for dental and jewelry uses.

Another object of the invention is to provide an alloy having sufficient ductility to enable dental constructions formed therefrom to be burnished or adjusted without fracturing.

A further object is to provide a silver based alloy having lowered work hardening characteristics whereby rolling of alloy bars into strips can be accomplished without excessive strain relief heat treatments.

A further object is to provide a silver based alloy having improved tarnish resistance, improved ductility and lessened work hardening characteristics without loss of properties necessary for casting, such as fluidity for filling a mold completely and for polishing.

Yet a further object is to provide a silver based alloy of improved characteristics for dental and jewelry uses comprising:

Element Broad range, Preferred range,

Pd 10-60 15-30 At least one of the group consisting of Cr, Fe, ln, Sn and Zn .2-10 .5-10

Ag Rest Rest Further objects of the invention will become apparent from the following description and claims.

DESCRIPTION OF THE INVENTION TARNISH RESISTANCE Publication No. 31,008 of the Department of Commerce dated Sept. 23, 1927 gives the results of tests on the tamishability of silver-based alloys. According to the publication, the alloys were exposed to sodium polysulfide for a period of 8 minutes and the discoloration noted. In the present case, this test was modified by substituting ammonium sulfide, which was found to give more reproducible results. In the corrosion tests employed, cast bar specimens were prepared by first grinding and polishing by normal metallurgical methods, followed by sand blasting the surface to be exposed to ammonium sulfide for 8 minutes.

A numerical scale was established by exposing pure silver bars to ammonium sulfide for times ranging from 0 to seconds, giving a scale of 0 for no exposure to 10, almost black. In making the test, a portion of the test bar was covered so that the exposed portion and unexposed portion could be directly compared. Dental 4 amalgam rated 9 on this scale, an alloy of 75 silver-25 TABLE II palladium rated 5. An ADA certified gold alloy rated between and l and was assigned '22. To be acceptable ADDED PERCENT ADDED. TARNISH RATING for dental work of the type covered in ADA specifica- ELEMENT tslOsl'l 5, the tarnish rating should be no higher than about 288: to 7% i Cobalt 7% 5 Chromium .2-2.9+ 2-5 HARDENING Copper 4 to 22 3-5 Iron 1-6 2-5 Elements in addition to copper may be employed to Indium up to 4% 1-2 harden a silver-based alloy containing palladium. A I l 2 1 5; partial list of elements is given in Table I, along with the 3235: 2 hardness obtained by adding them to an alloy contain- Tungsten 1 to 74 ing 75% Ag, 25% Pd.

TABLE I l 5 ADDED PERCENTAGE VICKERS HARDNESS It was found that in the case of some of the elements which in lower concentrations had the effect of improv- ELEMENT ADDED None 70 mg sulfide tarnlsh resistance had the opposite effect on Cobalt 10 120 higher concentrations. Thus, adding 25% indium to ggrg g produce an alloy having 135 Vickers rating resulted in Copper 22 245 unacceptable tarnish number of 7. 6 4 127 It has been discovered that employment in a silverermanlurn 2.2 103 H f i 4 104 palladium alloy of certain elements and combinations Indium 4% 160 thereof in specific ranges results in alloys having immgi gggsfi i 23 25 proved ductility, work hardening characteristics and Nickel 11 130 resistance to tarnishing. These elements are broadly El 1 3g chromium, iron, indium, tin and zinc and, more prefer- Titanium 140 ably indium, tin and zinc. Vanadium 01 92 Since hardness is not translatable from one alloy 2:25 1 g; system to another, some other mechanical property Zirconium 4 132 must be employed to compare different systems. In the present case, the 0.1% offset yield strength was used as the comparable property. The Vickers hardness shown Improved Sulfide tarnish resistance is gi n in a l in the table is for a silver base alloy of this invention II by the addition of the enumerated elements to a 35 having the same 0.1% offset yield strength as the ADA g, 25% Pd y: specification 5 Type alloys.

TABLE III TYPE 01% OFFSET YIELD VICKERS HARDNESS STRENGTH (ADA Specification No. 5) (Gold or Silver based alloy) (Silver base alloy) I 20900-21500 psi 235-105 H 27,50036,000 psi 105-145 Ill 36,000-44,500 psi 145-200 IV 44,500+ 200+ These values are all in the "as cast" condition.

Examples of alloys for Type I (HV -105) are listed in Table IV:

TABLE IV Alloy Cold No Ag Pd Cr Fe In Sn Zn HV Tamish Reduction 1 B211 2395 85 3 2 Bal 23.35 1 l 97 3 90%+ 3 Bal 23.50 I 105 2 90%+ 4 Ba! 25 2 86 2 90%+ 5 Hal 25 3 99 2 90%+ 6 Ba] 25 1 90 2 90%+ 7 Ba] 25 l 97 2 90%+ 8 Ba] 24.5 25 44 .33 .22 3 90%+ 9 Ba] 24.5 l l 91 5 90%+ I0 Bal 24.5 1 l 98 4 90%+ ll Bal 24.5 l l 90 4 90%+ Examples of alloys for Type II (HV -145) are listed in Table V:

TABLE V Allo Cold No. Ag Pd Cr Fe In Sn Zn HV Tamish Reduction 1 Bal 22.6 1.6 120 3 2 Bal 21.7 2.2 4 90%+ 3 Bal 20.65 2.9 126 4 90% 4 Bal 22 -2 3 90%|- 5 Bal l9 4 128 4 90%+ 6 Bal 25 3.5 120 2 90% 7 Bal 25 2 140 2 86% 8 Bal 25 2 2 87% 9 Bal 24.5 .25 l 113 3 89% 1O Bal 24 .5 .5 113 3 90%+ ll Bal 24 .5 2 114 3 90%+ l2 Bal 24 1 l 3 90% 13' Bal 24.25 2 l 4.5 90% 14 Bal 24 3 l 130 4 90%+ 15 Bal 24 2 l l 141 4 89% Examples of alloys for Table III (I-IV 145-200) are listed m Table VI: TABLE VIII continued TABLE VI Alloy Cold No. Ag Pd Cr Fe In Sn Zn HV Tamish Reduc.

l Bal l9 6 148 5 70% 2 Bal 25 4 150 1 89% 3 Hal 25 6 186 4.5 62% 4 Bal 25 4 4 58% 5 Bal 24.75 .25 1.1 .82 .55 162 3 89% 6 Bal 24.50 .5 .18 1.08 .72 173 3 80% 7 Bal 24.25 .75 .9 6 182 3 89% 8 Bal 24 1 1.1 .82 .55 191 4 77% 9 Bal 24.25 l l l 150 4.5 89% 10 Bal 24 1 l 2 160 4 65% ll Bal 23.75 2 3 4.5 51% 12 Bal 23.5 5 l 195 4 58% 13 Bal 23.25 4 3 184 4.5 56% 14 Bal 23 4 4 183 4.5 40% 15 Bal 22.5 l 4 5 155 5.5 35% Examples of alloys for Type IV (l-IV 200+) are listed RANGES OF ELEMENTS OF THE SILVER BASED ALLOY (PERCENT) Table VIII Cr, Fe, In, Sn, Zn .2-10 .540

TABLE VI] A110 Cold No. Ag Pd Cr Fe In Sn Zn HV Tamish Reduc.

1 Bal 25 4 206 4 86% 2 Ball 24.75 .25 1.76 1.32 .88 204 4 64% 3 Ball 23.75 4 l 218 4.5 71% 4 Hal 23.50 2 3 l 205 4 52% 5 Bal 23.25 6 1 219 4.5 52% 6 Bal 23 5 2 1 215 4 36% 7 Bal 22.5 6 4 202 4.5 37% 50 What is claimed is:

The following are the ranges of elements of the silver based alloy of the present invention:

1. An alloy consisting essentially of about 15-30% Pd, about 02-10% of an element of the group consisting of Cr, Fe, and In and mixtures thereof and the balance Ag.

2. The alloy of claim 1 wherein the content of the element of the group is about 05-10% and the balance Ag.

3. The alloy of claim 1 consisting essentially of about 23.5% Pd, about .1% Fe and the balance Ag.

4. The alloy of claim 1 consisting essentially of about 25% Pd, about 3.5% In and the balance Ag.

5. The alloy of claim 1 consisting essentially of about 24.25% Pd, about 0.75% Fe, about 0.9% Sn, about 6% Zn and the balance Ag.

6. The alloy of claim 1 further including from about 02-10% of an element of the group consisting of Sn and Zn and mixtures thereof. 

1. AN ALLOY CONSISTING ESSENTIALLY OF ABOUT 15-30% PD, ABOUT 0.2-10% OF AN ELEMENT OF THE GROUP CONSISTING OF CR, FE, AND IN AND MIXTURES THEREOF AND THE BALANCE AG.
 2. The alloy of claim 1 wherein the content of the element of the group is about 0.5-10% and the balance Ag.
 3. The alloy of claim 1 consisting essentially of about 23.5% Pd, about 1% Fe and the balance Ag.
 4. The alloy of claim 1 consisting essentially of about 25% Pd, about 3.5% In and the balance Ag.
 5. The alloy of claim 1 consisting essentially of about 24.25% Pd, about 0.75% Fe, about 0.9% Sn, about 6% Zn and the balance Ag.
 6. The alloy of claim 1 further including from about 0.2-10% of an element of the group consisting of Sn and Zn and mixtures thereof. 